Solid-state lithium batteries represent a transformative advancement in energy storage technology, offering the potential for high energy density, enhanced safety, and long cycle life compared to conventional liquid electrolyte systems. However, the practical implementation of solid-state batteries is hindered by significant challenges at the electrode-electrolyte interface, including high interfacial resistance, poor physical contact, and the growth of lithium dendrites. These issues stem from the rigid solid-solid interactions between the electrolyte and electrodes, which impede efficient lithium-ion transport and lead to performance degradation. In this context, in-situ polymerization has emerged as a pivotal strategy to address these interfacial problems by enabling the formation of conformal, integrated interfaces through a liquid-to-solid transformation process. This approach not only reduces interfacial impedance but also enhances mechanical adhesion and simplifies the manufacturing process, making it highly suitable for scalable production of solid-state batteries.
The core principle of in-situ polymerization involves the application of a precursor solution—comprising monomers, initiators (in some cases, initiator-free systems), lithium salts, and optional additives—directly onto the electrode surfaces. Upon triggering by external stimuli such as ultraviolet (UV) light or heat, the solution undergoes polymerization to form a solid polymer electrolyte (SPE) that intimately infiltrates the electrode pores. This results in a seamless interface that facilitates rapid ion transport and mitigates side reactions. Unlike ex-situ fabricated electrolytes, which often suffer from poor wettability and thick, brittle membranes, in-situ formed electrolytes can achieve thin, flexible, and robust layers with tailored properties. The selection of polymer matrices plays a critical role in determining the electrochemical performance of the solid-state battery. Key categories include ether-based polymers (e.g., poly(ethylene oxide), PEO), ester-based polymers (e.g., acrylates and carbonates), fluoride-based polymers (e.g., poly(vinylidene fluoride), PVDF), and ionic liquid-based polymers, each offering distinct advantages in terms of ionic conductivity, mechanical strength, and interfacial stability.
In this comprehensive review, we delve into the recent advancements in in-situ polymer electrolytes for solid-state lithium batteries, with a focus on interfacial optimization strategies. We explore the synthesis methodologies, electrochemical properties, and mechanistic insights across different polymer systems, supported by comparative tables and theoretical models. The integration of functional additives, fillers, and multilayer designs is examined to highlight their role in enhancing ionic transport, suppressing dendrite growth, and extending cycle life. Furthermore, we discuss the prospects and challenges of in-situ polymerization techniques in enabling high-performance solid-state batteries for applications in electric vehicles, portable electronics, and grid storage. By emphasizing key terms such as “solid state battery” and “solid state batteries” throughout, we aim to provide a detailed resource for researchers and engineers working in this rapidly evolving field.
Ether-Based Polymer Electrolytes
Ether-based polymers, particularly PEO and its derivatives, have been extensively studied for solid-state batteries due to their high solvation capacity for lithium salts and good compatibility with lithium metal anodes. However, their low ionic conductivity at room temperature (typically below 10−4 S/cm) and limited electrochemical stability window (∼3.8 V) pose challenges for high-voltage applications. In-situ polymerization strategies have been employed to overcome these limitations by improving interfacial contact and incorporating functional additives. For instance, PEO-based electrolytes fabricated via casting methods can form cathode-supported structures that enhance adhesion and active material utilization. In such systems, the ionic conductivity (σ) can be described by the Arrhenius equation: $$ \sigma = \sigma_0 \exp\left(-\frac{E_a}{kT}\right) $$ where σ0 is the pre-exponential factor, Ea is the activation energy, k is Boltzmann’s constant, and T is the temperature. The addition of lithium salts like LiTFSI or LiDFOB not only improves σ but also promotes the in-situ formation of stable solid electrolyte interphase (SEI) layers, which suppress side reactions and reduce interfacial resistance.
Cyclic ethers, such as 1,3-dioxolane (DOL), can undergo ring-opening polymerization to form poly-DOL (PDOL) electrolytes with enhanced mechanical properties and higher ionic conductivity. The polymerization kinetics follow a first-order reaction model: $$ \frac{d[M]}{dt} = -k_p [M][I] $$ where [M] is the monomer concentration, [I] is the initiator concentration, and kp is the rate constant. Surface-initiated polymerization on substrates like glass can achieve high conversion rates (e.g., 97.6%) without external initiators, leading to robust interfaces in solid-state batteries. The lithium-ion transference number (tLi+), a critical parameter for minimizing concentration polarization, is often enhanced in these systems through the use of dual-salt formulations or plasticizers like succinonitrile (SN). For example, tLi+ can be calculated using the Bruce-Vincent method: $$ t_{Li+} = \frac{I_s(\Delta V – I_0 R_0)}{I_0(\Delta V – I_s R_s)} $$ where I0 and Is are the initial and steady-state currents, ΔV is the applied voltage, and R0 and Rs are the initial and steady-state resistances.
| Polymer Type | Ionic Conductivity (S/cm) | tLi+ | Electrochemical Window (V) | Key Additives/Fillers | Performance Highlights |
|---|---|---|---|---|---|
| PEO/LiTFSI | 10−4 at 25°C | 0.2–0.3 | ∼3.8 | LiDFOB, SN | Stable cycling in LiFePO4 batteries; capacity retention >80% after 1000 cycles |
| PDOL/LiPF6 | 10−3 at 25°C | 0.5–0.6 | ∼4.5 | None (surface-initiated) | Symmetrical Li cells stable for 4000 h at 0.3 mA/cm2 |
| PEO/LLZO Composite | 10−3 at 60°C | 0.4–0.5 | ∼4.2 | Ga-LLZO, LiBOB | High-voltage compatibility with NCM622 cathodes |
Multilayer designs, such as bilayer electrolytes with cathode-side modifications, have been developed to extend the electrochemical stability window of ether-based systems. For instance, a PEO/LiDFOB layer on the cathode side can form a protective CEI rich in LiF and B-O species, enabling operation at voltages up to 4.2 V. The interfacial resistance (Rint) in such systems is modeled by: $$ R_{int} = \frac{\delta}{\sigma_{int}} $$ where δ is the interface thickness and σint is the interfacial conductivity. By optimizing the polymer architecture and salt composition, ether-based in-situ electrolytes can achieve tLi+ values above 0.5 and ionic conductivities exceeding 1 mS/cm at elevated temperatures, making them suitable for a wide range of solid-state battery applications.
Ester-Based Polymer Electrolytes
Ester-based polymers, including acrylates and carbonates, offer higher ionic conductivities and better mechanical strength than ether-based systems, owing to their amorphous nature and higher dielectric constants. In-situ polymerization of monomers like ethoxylated trimethylolpropane triacrylate (ETPTA), poly(ethylene glycol) diacrylate (PEGDA), and vinyl ethylene carbonate (VEC) is typically initiated by UV light or heat, resulting in cross-linked networks that enhance electrolyte stability. The ionic conductivity in these polymers is influenced by the segmental motion of the polymer chains, which can be described by the Vogel-Tammann-Fulcher (VTF) equation: $$ \sigma = A T^{-1/2} \exp\left(-\frac{B}{T – T_0}\right) $$ where A and B are constants, and T0 is the glass transition temperature. For example, in-situ UV-polymerized PEGDA electrolytes with ceramic fillers like LLZO can achieve σ values of 1.33 mS/cm at 25°C and tLi+ of 0.89, along with a wide electrochemical window of 5.5 V.
Acrylate-based systems often incorporate functional additives to improve interfacial properties. For instance, TFEMA monomers introduce fluorinated groups that enhance oxidative stability and promote the formation of LiF-rich SEI layers. The effectiveness of these additives in suppressing dendrite growth can be quantified by the Sand’s time model: $$ \tau = \frac{\pi D \left(\frac{e C_0}{2 \mu}\right)^2}{J^2} $$ where D is the diffusion coefficient, C0 is the initial Li+ concentration, e is the electron charge, μ is the mobility, and J is the current density. In carbonate-based systems, in-situ polymerization of VEC with cross-linkers like PETEA creates gel polymer electrolytes (GPEs) that exhibit high σ (6.9 mS/cm) and good adhesion to electrodes. The incorporation of LiBOB salts further stabilizes the interface by forming B-O and B-F containing CEI layers, which reduce side reactions and improve cycle life in high-voltage solid-state batteries.
| Monomer System | Polymerization Method | σ (S/cm) at 25°C | tLi+ | Mechanical Strength (MPa) | Application in Solid-State Batteries |
|---|---|---|---|---|---|
| ETPTA/PEGDA | UV curing | 10−3 | 0.7–0.8 | 5–10 | Li metal batteries with NCM811; stable for 500 cycles |
| VEC/PETEA | Thermal polymerization | 6.9 × 10−3 | 0.51 | 3–5 | LCO full cells; 83.4% capacity retention after 100 cycles |
| PEGMEA/LLZO | In-situ casting | 3.48 × 10−2 | 0.89 | 12 | Ultra-thin electrolytes (10 μm) for flexible batteries |
To address the low mechanical strength of ester-based electrolytes, composite structures with fibrous scaffolds (e.g., cellulose, PI membranes) or ceramic fillers (e.g., LAGP, Li3InCl6) are employed. These materials not only reinforce the electrolyte matrix but also create continuous ion-conduction pathways. The overall conductivity of composite electrolytes can be estimated using the effective medium theory: $$ \sigma_{eff} = \sigma_m \phi_m + \sigma_f \phi_f $$ where σm and σf are the conductivities of the matrix and filler, and φm and φf are their volume fractions. Asymmetric designs, such as rigid ceramic layers on the anode side and soft polymer layers on the cathode side, have been shown to reduce internal resistance and prevent dendrite penetration, enabling long-cycle-life solid-state batteries with high energy density.
Fluoride-Based Polymer Electrolytes
Fluoride-based polymers, such as PVDF and its copolymers (e.g., PVDF-HFP), are valued for their high electrochemical stability and mechanical toughness, making them suitable for high-voltage solid-state batteries. However, their inherent low ionic conductivity requires modification through in-situ polymerization with ionic liquids (ILs) or ceramic fillers. For example, in-situ formed PVDF-HFP/IL/LAGP composite electrolytes can achieve σ values of 0.51 mS/cm at 30°C and form stable SEI layers rich in LiF, which enhance interface compatibility. The ion transport in these systems is governed by the Nernst-Einstein equation: $$ \sigma = \frac{n e^2 D}{kT} $$ where n is the charge carrier density, e is the elementary charge, and D is the diffusion coefficient.
Quasi-double-layer structures have been developed to tailor the interface properties of fluoride-based electrolytes. In such designs, the anode-side layer incorporates reducible solvents like diglyme (DGM) to form a protective SEI, while the cathode-side layer uses oxidatively stable solvents like PC that polymerize in-situ to form a CEI. The interfacial stability can be evaluated by the exchange current density (i0), given by: $$ i_0 = \frac{RT}{nFR_{ct}} $$ where R is the gas constant, T is temperature, n is the number of electrons, F is Faraday’s constant, and Rct is the charge-transfer resistance. These strategies have enabled fluoride-based solid-state batteries to achieve capacity retention of 80.2% after 200 cycles in NCM811 systems, demonstrating their potential for high-energy applications.
| Polymer Matrix | Additive/Plasticizer | σ (S/cm) at 25°C | Electrochemical Window (V) | Cycle Life (Cycles) | Notable Features |
|---|---|---|---|---|---|
| PVDF-HFP | IL (EmimFSI) | 5.1 × 10−4 | 4.5–5.0 | 196 (LiFePO4) | Flame-retardant; stable SEI formation |
| PVDF/LLZTO | PC/DGM | 10−3 | 5.3 | 200 (NCM811) | Quasi-double-layer; low Rint |
| PVDF/PMMA Blend | SN | 2 × 10−3 | 4.8 | 300 (LCO) | High mechanical flexibility |
Advanced fabrication techniques, such as electrospinning or in-situ casting-drying, have been used to produce ultra-thin (20 μm) fluoride-based electrolytes with low resistance and high flexibility. These electrolytes facilitate the use of high-capacity electrodes in solid-state batteries, such as silicon oxide anodes and NCM cathodes, and have been demonstrated in pouch cell configurations with stable cycling over 600 cycles. The continued optimization of fluoride-based systems through molecular design and interface engineering is crucial for realizing their full potential in commercial solid-state battery technologies.
Ionic Liquid-Based Polymer Electrolytes
Polymerized ionic liquids (PILs) combine the high ionic conductivity and thermal stability of ionic liquids with the mechanical integrity of polymers, making them ideal for solid-state batteries operating under harsh conditions. In-situ polymerization of IL monomers, such as diallyldimethylammonium TFSI (DADMA-TFSI) or vinyl-imidazolium salts, creates cross-linked networks that exhibit σ values up to 0.7 mS/cm and wide electrochemical windows (4.9–5.6 V). The ion transport in PILs is characterized by a high tLi+ (e.g., 0.91 in some systems), which reduces polarization and improves rate capability. The transference number can be expressed as: $$ t_{Li+} = \frac{\sigma_{Li+}}{\sigma_{total}} $$ where σLi+ is the lithium-ion conductivity and σtotal is the total ionic conductivity.
In-situ UV polymerization strategies have been employed to fabricate PIL-based electrolytes with semi-interpenetrating networks (semi-IPNs) that enhance mechanical strength and interface adhesion. For example, PIL fibers cross-linked with PTFEMA form ionogel electrolyte membranes (IGEMs) that maintain high conductivity (0.7 mS/cm) across a broad temperature range (0–90°C). The thermal stability of these electrolytes is quantified by the degradation temperature (Td), which often exceeds 300°C due to the robust covalent bonds in the polymer matrix. Additionally, the incorporation of PEG segments into PIL networks improves flexibility and compatibility with lithium metal anodes, enabling solid-state batteries with long cycle life and high safety.
| PIL Monomer | Polymerization Trigger | σ (S/cm) at 25°C | tLi+ | Thermal Stability (°C) | Battery Performance |
|---|---|---|---|---|---|
| DADMA-TFSI | UV light | 7 × 10−4 | 0.91 | 350 | LiFePO4 cells; 153 mAh/g at 0.2C |
| Vinyl-Imidazolium | Heat | 5 × 10−3 | 0.75 | 320 | Wide-temperature operation; 1000 cycles |
| PEG-PIL Copolymer | UV curing | 10−3 | 0.8 | 300 | High-capacity cathodes; 90% retention |
The design of PIL electrolytes often involves functionalization with groups like -CH2CF3 to regulate Li+ solvation and transport. Molecular dynamics simulations indicate that these groups create preferential pathways for Li+ migration, thereby increasing σ and tLi+. Furthermore, the use of IL-based plasticizers, such as EmimFSI, reduces crystallinity and enhances segmental motion, leading to improved low-temperature performance. These advancements position PIL-based electrolytes as promising candidates for next-generation solid-state batteries that require high energy density, safety, and operational reliability.
Interfacial Optimization Strategies
Interfacial optimization is paramount for the success of in-situ polymer electrolytes in solid-state batteries. Key strategies include the use of sacrificial additives, multilayer architectures, and surface modifications to create stable, low-resistance interfaces. Additives like LiPO2F2, LiNO3, and I2 decompose during cycling to form inorganic-rich SEI/CEI layers composed of LiF, LixByOz, or other compounds that inhibit side reactions and dendrite growth. The effectiveness of these layers can be modeled using interface resistance equations and electrochemical impedance spectroscopy (EIS) data. For instance, the total cell resistance (Rcell) in a solid-state battery is given by: $$ R_{cell} = R_{bulk} + R_{int} + R_{ct} $$ where Rbulk is the bulk electrolyte resistance, Rint is the interfacial resistance, and Rct is the charge-transfer resistance.
Multilayer electrolyte designs, such as polymer/ceramic/polymer sandwiches, leverage the strengths of different materials to achieve balanced properties. The ceramic layer (e.g., LLZO or LAGP) provides mechanical rigidity and dendrite suppression, while the polymer layers ensure flexible contact with electrodes. The overall ionic conductivity in such composites can be approximated by a series resistance model: $$ \frac{1}{\sigma_{total}} = \frac{\phi_1}{\sigma_1} + \frac{\phi_2}{\sigma_2} + \frac{\phi_3}{\sigma_3} $$ where φi and σi are the volume fractions and conductivities of each layer. Asymmetric interfaces, with tailored properties on the anode and cathode sides, have been shown to reduce Rint by up to 50%, enabling solid-state batteries with high rate capability and long cycle life.
Surface engineering techniques, such as plasma treatment or chemical functionalization of electrodes, improve wettability and adhesion of in-situ electrolytes. For example, cellulose-based scaffolds with polar groups enhance Li+ transport and mechanical integrity, leading to CPEs with σ > 1 mS/cm and tensile strengths of 4.5 MPa. These approaches, combined with in-situ polymerization, facilitate the development of solid-state batteries that meet the demanding requirements of commercial applications, including electric vehicles and grid storage systems.
Conclusions and Future Perspectives
In-situ polymerization has revolutionized the design of polymer electrolytes for solid-state lithium batteries by enabling seamless integration with electrodes and optimizing interfacial properties. Through the careful selection of polymer matrices—such as ether-based, ester-based, fluoride-based, and ionic liquid-based systems—researchers have achieved significant improvements in ionic conductivity, lithium-ion transference number, and electrochemical stability. The incorporation of functional additives, ceramic fillers, and multilayer structures further enhances performance by stabilizing interfaces and suppressing dendrite growth. These advancements have led to solid-state batteries with high energy density, excellent safety, and long cycle life, as demonstrated in various cell configurations including pouch cells and high-voltage systems.
Despite these progress, challenges remain in scaling up in-situ polymerization processes for commercial solid-state batteries. Issues such as monomer reactivity, polymerization kinetics, and cost-effectiveness need to be addressed through continued research. Future work should focus on developing new monomer systems with enhanced stability, exploring hybrid electrolytes that combine the advantages of different polymers, and optimizing interface engineering for ultra-thin electrolytes. Additionally, in-depth studies on the degradation mechanisms and safety profiles under realistic operating conditions are essential for the widespread adoption of these technologies. With ongoing innovations, in-situ polymer electrolytes are poised to play a pivotal role in the next generation of solid-state batteries, driving advancements in energy storage for a sustainable future.